Operational Amplifiers

1. Electronic Devices and Circuit Theory Boylestad Operational Amplifiers Chapter 10

2. Ch.10 Summary The Basic Op-Amp Operational amplifier (Op-amp):A high gain differential amplifier with a high input impedance (typically in M) and low output impedance (less than 100). Note the op-amp has two inputs and one output.

3. Ch.10 Summary Op-Amp Gain • Op-Amps can be connected in open-loop or closed-loop configurations. • Open-loop: A configuration with no feedback from the op-amp output back to its input. Op-amp open-loop gain typically exceeds 10,000. • Closed-loop: A configuration that has a negative feedback path from the op-amp output back to its input. Negative feedback reduces the gain and improves many characteristics of the op-amp. • Closed-loop gain is always lower than open-loop gain.

4. Ch.10 Summary Inverting Op-Amp The input signal is applied to the inverting (–) input The non-inverting input (+) is grounded The feedback resistor (Rf)is connected from the output to the negative (inverting) input; providing negative feedback.

5. Ch.10 Summary Inverting Op-Amp Gain Gain is set using external resistors: Rf and R1 Gain can be set to any value by manipulating the values of Rf and R1. Unity gain (Av = 1): The negative sign denotes a 180 phase shift between input and output.

6. Ch.10 Summary Virtual Ground Virtual ground: A term used to describe the condition where Vi 0 V (at the inverting input) when the noninverting input is grounded. The op-amp has such high input impedance that even with a high gain there is no current through the inverting input pin, therefore all of the input current passes through Rf.

7. Ch.10 Summary Common Op-Amp Circuits Inverting amplifier Noninverting amplifier Unity follower Summing amplifier Integrator Differentiator

8. Ch.10 Summary Inverting/Noninverting Amplifiers Inverting Amplifier Noninverting Amplifier

9. Ch.10 Summary Unity Follower

10. Ch.10 Summary Summing Amplifier Because the op-amp has a high input impedance, the multiple inputs are treated as separate inputs.

11. Ch.10 Summary Integrator The output is the integral of the input; i.e., proportional to the area under the input waveform. This circuit is useful in low-pass filter circuits and sensor conditioning circuits.

12. Ch.10 Summary Differentiator The differentiator takes the derivative of the input. This circuit is useful in high-pass filter circuits.

13. Ch.10 Summary DC-Offset Parameters Even when the input voltage is zero, an op-amp can have an output offset. The following can cause this offset: • Input offset voltage • Input offset current • Input offset voltage and input offset current • Input bias current

14. Ch.10 Summary Input Offset Voltage (VIO) The specification sheet for an op-amp indicates an input offset voltage (VIO). The effect of this input offset voltage on the output can be calculated with

15. Ch.10 Summary Input Offset Current (IIO) If there is a difference between the dc bias currents generated by the same applied input, this also causes an output offset voltage: • The input offset current (IIO) is specified in the specifications for an op-amp. • The effect of IIO on the output offset voltage can be calculated using:

16. Ch.10 Summary Total Offset Due to VIO and IIO Op-amps may have an output offset voltage due to VIO and IIO. The total output offset voltage equals the sum of the effects of both:

17. Ch.10 Summary Input Bias Current (IIB) A parameter that is related to input offset current (IIO) is called input bias current(IIB) The input bias currents are calculated using: The total input bias current is the average of the two:

18. Ch.10 Summary Frequency Parameters An op-amp is a wide-bandwidth amplifier. The following factors affect the bandwidth of the op-amp: • Gain • Slew rate

19. Ch.10 Summary Gain and Bandwidth The op-amp’s high frequency response is limited by its internal circuitry. The plot shown is for an open loop gain (AOL or AVD). This means that the op-amp is operating at the highest possible gain with no feedback resistor. In the open loop mode, an op-amp has a narrow bandwidth. The bandwidth widens in closed-loop mode, but the gain is lower.

20. Ch.10 Summary Slew Rate (SR) Slew rate (SR): The maximum rate at which an op-amp can change output without distortion. The SR rating is listed in the specification sheets as the V/s rating.

21. Ch.10 Summary Maximum Signal Frequency The slew rate determines the highest frequency of the op-amp without distortion. where VP is the peak voltage

22. Ch.10 Summary General Op-Amp Specifications Other op-amp ratings found on specification sheets are: • Absolute Ratings • Electrical Characteristics • Performance

23. Ch.10 Summary Absolute Ratings These are common maximum ratings for the op-amp.

24. Ch.10 Summary Electrical Characteristics Note: These ratings are for specific circuit conditions, and they often include minimum, maximum and typical values.

25. Ch.10 Summary CMRR One rating that is unique to op-amps is CMRR or common-mode rejection ratio. Because the op-amp has two inputs that are opposite in phase (inverting input and the non-inverting input) any signal that is common to both inputs will be cancelled. Op-amp CMRR is a measure of the ability to cancel out common-mode signals.

26. Ch.10 Summary Op-Amp Performance The specification sheets will also include graphs that indicate the performance of the op-amp over a wide range of conditions.